It is an empty class, and as long as they implement the methods defined as an Server adapter interface, they do not need to inherit Plack::Handler.

If you write a new handler for existing web servers, I recommend you to include the full name of the server module after Plack::Handler prefix, like Plack::Handler::Net::Server::Coro if you write a handler for Net::Server::Coro. That way you'll be using plackup command line option like:

plackup -s Net::Server::Coro

that makes it easy to figure out which web server you're going to use.